The present invention relates to an automatic electronic span circuit including means for automatically adjusting the gain of an amplifier to make its output correspond to a predetermined value. The invention is particularly useful in connection with programmed analytical instruments where standard samples of known compositions are periodically introduced into the instrument as calibration checks.
In many field applications of analytical instruments, it is important to automatically change the gain of the analyzer amplifier to make the output voltage or current agree with the known composition of a reference sample. This is usually accomplished by means of a servo system which drives a gain control, or attenuator, potentiometer to a point such that the output agrees with a preselected reference voltage. For example, a 68 ppm (parts per million) standard sample may be used where the full scale span of the analyzer is 100 ppm. A reference voltage of 68% of full scale may then be set so that when the standard sample is introduced into the analyzer the span servo is programmed on to set the gain control to a point where the output is precisely 68% of full scale. Then, when the servo is turned off, the gain setting remains until the next programmed span adjustment. The major disadvantages of the servo system are the initial cost and high maintenance level due primarily to the wear of the mechanical parts.
The present invention provides an automatic span circuit which electronically performs the same function upon the output of an analyzer as does the servo system, but with no moving parts. The gain of an analyzer amplifier is adjusted or controlled by a digital signal generated by the span circuit in such a manner that the output of the amplifier from a given standard sample is made to be equal to a given reference voltage. The digital circuit, when deactivated, maintains a status quo condition such that the gain set immediately prior to deactivation is retained until the next setting or "span" cycle. A direct current electrical signal voltage generated by a sample analyzer is fed to the analog input of a conventional multiplying digital-to-analog converter (MDAC), the analog output of which is connected to one input of an analog comparator and compared to a reference voltage connected to the other comparator input. The reference voltage, which may be obtained from the adjustable contact of a potentiometer connected across a battery, is set at a voltage less than the output of the analyzer for the standard sample, since the gain of the MDAC is less than 1. The comparator generates at its output one of two voltage levels depending on whether the MDAC output voltage is larger or smaller than the reference voltage. The comparator output is connected to two electronic gates which selectively transmit voltage pulses from a pulse generator to the UP or DOWN inputs of a binary UP/DOWN counter having its digital output counting channels connected, respectively to the digital input channels of the MDAC in such manner that the UP/DOWN counter counts UP when the MDAC output voltage is smaller than the reference voltage, and counts DOWN when the MDAC output voltage is larger than the reference voltage. These digital inputs to the MDAC automatically adjust the gain of the MDAC amplifier, up or down, until the MDAC output voltage is equal to the reference voltage. In a short time, the calibration of the MDAC to the particular reference voltage used is completed, and the electronic gates are disabled, manually or automatically. The MDAC remains connected to the UP/DOWN counter, and hence, retains the final digital input and gain adjustment. A visual and/or recording voltmeter, connected between the MDAC output and ground, is calibrated to read the known input voltage to the MDAC, or the known concentration in ppm of the standard sample, instead of the actual output voltage. The actual output voltage is the actual input voltage x the fixed fractional gain of the MDAC.
When the circuit described is then used to measure an unknown sample, the output voltage thereof, or concentration in ppm, is indicated directly by the calibrated meter.
At any time thereafter, the automatic span circuit can be recalibrated, by introducing the same or a different standard sample of known concentration adjust the gain of the MDAC, and re-calibrating the meter, as in the initial calibration.